It is known to produce metal parts by compacting powdered metal and heating the compacted metal powder to sinter the metal particles to produce a finished metal product. However, such products are known to have lower densities than comparable wrought metal products. To achieve greater densities, products have been subjected to secondary operations, such as using a second coining operation.
Recently, there has been interest in the use of a compacting press to manufacture metal parts from metal powders where the metal particles are coated with a thin polymer lubricant. The polymer coatings are believed to lubricate the interfaces between metal particles and forming tool surfaces thereby making it easier to form workpieces due to decreases in the internal friction and stresses created during compaction. First level workpieces using conventional lubricated powders have been compacted into workpieces having final densities in the range of 6.8-7.1 grams per cubic centimeter (g/cc) when compacting at pressures of 35-60 TSI. To obtain higher densities, over 7.2 g/cc, a workpiece is typically first green-formed using lubricated powders to a density of about 6.8, and then pre-sintered to a temperature typically between 1400 and 1700 degrees Fahrenheit (.degree. F.) in order to burn out the lubricants, relieve internal stresses, and to promote mechanical bonding. Next, the workpiece surface is coated with a lubricant and pressed a second time to reach the preferred density range. As can be appreciated with such level 2 and 3 workpieces, reaching higher density levels becomes costly and time-consuming; moreover, there may be density fluctuations in the workpiece after the second pressing. And while advancements in the use of lubricants and other materials have allowed for production of workpieces with increased densities without resorting to secondary operations, pressing a room-temperature lubricated powder to a density in the range of 7.1 g/cc has its price: for example, the compressive forces are typically very high in such operations, on the order of at least 100,000 psi, and the wear of the tools is accelerated; moreover, with the much higher compressive forces needed to reach these densities, the workpieces can develop surface irregularities from the extreme internal stresses.
More recent discoveries have found that pre-heating the polymer lubricated products has helped not only to obtain higher densities at lower compacting pressures, but also to achieve homogeneous workpiece densities. Homogeneous workpiece density levels greater than 7.4 g/cc can be obtained. Generally, by using warm lubricated metal powders, it has been possible to increase densities by about 0.2 g/cc while eliminating the second coining or pressing operation.
However, problems have arisen with the apparatus necessary to heat the polymer-coated metal powder and deliver the warm powder to the die in the compacting press. When heated to a temperature just below a critical powder temperature (about 300 degrees Fahrenheit) prior to loading the material into the die cavity, the heated powder may become tacky and coagulate. Typical powder delivery systems have used flexible, accordion-pleated hoses for transferring powder to the die cavity; but when such hoses are used with warm powder, the powder may gather in the interstices of the pleats, become compacted and block the hose. Since the usual production method requires accurate gravity-controlled filling of the powder into the die cavity from a delivery shoe, any coagulation or tackiness of the coated powder can cause variations in the amount of powder that is actually deposited in the die cavity. Other problems may result from the tacky powder creating blockage in the powder delivery system. In addition, conventional flexible hoses and seals may degrade or be destroyed when exposed to the elevated temperature of the warm powder.
One attempt to solve these problems in the art is shown in U.S. Pat. No. 5,213,816 (1993) to Smyth, Jr. et al. In that patent, which discloses an auger conveyor assembly and a shuttle assembly, the auger assembly has a horizontal tube with an interior auger which is fed powdered metal through a vertical tube. These parts are heated. The discharge end of the heated horizontal tube overlies a heated hopper in the shuttle assembly, and the heated metal powder is dropped into the shuttle hopper and to a heated ring. This design has problems associated with its operation. First, the heated powder drops through the air from the discharge end of the horizontal tube to the hopper, allowing the metal powder to cool, and in the case of mixtures of different metal powders, to separate into a less homogeneous mixture. In addition, to change the machine over from one type of metal powder to another may be difficult and time-consuming. Changing the die in the compacting press may also be difficult because of the presence of the shuttle assembly.